Mario and Wario came back from their previous galaxy, when the Mailtoad walked up to them.
Mario: Oh, hi Mailtoad! What do you have for us now?
Mailtoad: You have a letter from Luigi and Waluigi!
Wario takes the letter.
Wario: Let me read that.
The letter said:
Bros! We got a Star.
But now we can't get back! This picture shows where we are. Help us!
From Luigi and Waluigi
Wario: Our brothers are trapped in a galaxy again! This time it’s the Battlerock Galaxy, under a saucer.
Mario: They are? Let's-a-go rescue them!
Wario: Right behind you!
Mario and Wario quickly make their way to the Fountain Dome, then they blast off to the Battlerock Galaxy. They collect all 5 blue star shards again, using the appearing Pull Stars to keep going. Soon they arrive at the last saucer, which starts moving as the Battlerock's cannons activate and begin shooting.
Wario: Oh come on! Now we gotta go through this again?!
Mario: Would you rather leave Luigi and Waluigi to rot inside that container?
Wario: You're right. Let's go.
Mario and Wario ready themselves as the saucer approaches the Battlerock's cannon. They use Star Bits to destroy some of the cannonballs that almost hit them, while at the same time avoiding walls of lasers by switching between both sides of the saucer. Eventually they come to where Luigi and Waluigi are.
Luigi: Oh good, Bro, you made it! Now get us out of here.
Waluigi: We got a surprise for both of you!
Mario: Of course you have something, but it's no surprise that it's a Star!
Meanwhile, a Bullet Bill is chasing Wario over to the glass container, then the Bullet Bill smashes the container to pieces.
Luigi: Thanks, Wario! But really, thank you both.
Waluigi: Wario did all the work. Mario, you just stood there.
Mario: Doesn't matter! Now we can grab that Star together and go home!
Luigi and Waluigi present the Star. To Mario and Wario's surprise, it is...
Mario and Wario (in unison): A Green Star!
Waluigi: Still think it's not a surprise, Mario?
Mario: Who cares? Now we're one step closer to the Trial Galaxies!
Luigi: That's right!
Wario: Let's go then!
The quartet of Brothers all grab the Green Star together, with Mario and Wario in front and the Luigis in back.
All 4 of them (in unison):
On to the next Star!